1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method, and, more particularly to an image forming apparatus and an image forming method for performing color printing and monochrome printing according to an electrophotographic system.
2. Description of the Related Art
When color printing is performed using the electrophotographic system, a method of superimposing images of four colors, Y (Yellow), M (Magenta), C (Cyan), and K (black), to form a full-color image is often used. As a system for forming color images of four colors and superimposing the color images, there are a four-rotation system, a tandem system, a system that uses plural transfer drums, and the like.
As the method of forming color images using plural transfer drums, for example, as disclosed in U.S. Pat. No. 5,087,945, there is a system that uses four image forming units and two transfer drums. In this system, images (e.g., a Y image and an M image) formed by two image forming units, respectively, are superimposed on each other to form a first two-color image (a Y+M image) on a first transfer drum. On the other hand, images (e.g., a C image and a K image) formed by the other two image forming units, respectively, are superimposed on each other to form a second two-color image (a C+K image) on a second transfer drum. The first two-color image is transferred from the first transfer drum onto the second transfer drum to form a full-color image (a Y+M+C+K image) on the second transfer drum. Thereafter, the full-color image on the second transfer drum is transferred onto recording paper.
Besides, an image forming apparatus that uses four image forming units and three transfer drums is also known. The transfer drums of this system include two first-stage transfer drums (first and second transfer drums) and one second-stage transfer drum (a third transfer drum). Two-color images (e.g., a Y+M image and a C+K image) are transferred onto the two first-stage transfer drums, respectively, from two image forming units corresponding thereto. The two-color images on the first and second transfer drums are transferred to be superimposed on each other on the second-stage transfer drum (the third transfer drum) to form a full-color image (a Y+M+C+K image) on the third transfer drum. Thereafter, the full-color image is transferred onto recording paper.
The system that uses two or three transfer drums (hereafter referred to as plural transfer drum system) and the tandem system are systems for forming plural color images substantially simultaneously to superimpose the color images one on top of another. Compared with the four-rotation system for sequentially forming each color image in every rotation of a transfer drum to superimpose color images formed one on top of another, it is possible to print a color image at high speed.
On the other hand, when monochrome images are printed using color image forming apparatuses of these systems, a system for using only an image forming unit for one color of black (K) has been often used.
When monochrome printing is performed by the color image forming apparatus of the four-rotation system, since an image of K is formed in one rotation of the transfer drum, in principle, printing speed is improved by four fold compared with the color printing.
On the other hand, since the plural transfer drum system and the tandem system are originally systems for forming plural color images substantially simultaneously to superimpose the color images one on top of another, printing speed is not very different in the color printing for plural colors (e.g., four colors) and the monochrome printing for one color.
When it is attempted to improve speed of the monochrome printing in the image forming apparatus of the plural transfer drum system and the tandem system in the past, it is inevitable to substantially change types and structures of components used in the image forming apparatuses and details of image formation processing during the color printing and during the monochrome printing. This makes it inconvenient for users to handle the apparatuses and causes an increase in cost of the apparatuses.